This is normal mirror lockup behavior, not necessarily a fault. On the Canon 80D, when shooting through the viewfinder with mirror lockup enabled, you must press the shutter button twice: the first press raises and locks the mirror, and the second press actually fires the shutter and takes the photo. While the camera is waiting for that second press, most other controls will not respond; typically only the shutter button and the power switch work. If you do not press the shutter again within about 30 seconds, the mirror will return to its normal position. If Live View and normal viewfinder shooting work correctly, that strongly suggests the camera is behaving as designed rather than failing.